This invention relates to the field of electrical energy storage and, more particularly, to a method and apparatus for quick or fast charging sealed nickel-cadmium batteries without risk of battery damage.
Sealed nickel-cadmium batteries are widely used in industry and by consumers as a source of electrical energy because they are rechargeable and otherwise require no maintenance. The charging rate of a battery is normally expressed as a multiple of its capacity (C) in, for example, ampere-hours. In other words, a cell charged at the C rate in amperes will become fully charged in one hour.
Nickel-cadmium batteries are charged at widely different rates ranging from 0.01 C to 10 C or more. A charging rate of 0.01 C to 0.04 C, regarded as standby charging, is utilized to maintain a battery in its fully charged state after it has been charged at a faster rate. A charging rate of 0.05 C to 0.1 C, regarded as slow charging, is utilized to charge a discharged battery without charge control, because a slow charging current can be applied to any sealed nickel-cadmium battery for an indefinite period of time after a fully charged state has been reached without damaging the battery. Slow charging requires from ten to twenty hours to charge a fully discharged battery. A charging rate of 0.2 C to 0.3 C, regarded as quick charging, is utilized to charge specially designed sealed nickel-cadmium batteries without charge control, because these specially designed batteries are able to handle quick charging current in overcharge without excessive pressure buildup or temperature rise. Quick charging requires from three to five hours. A charging rate above 0.3 C up to and exceeding 10 C, regarded as fast charging, is generally utilized to charge sealed nickel-cadmium batteries only with charge control, because the pressure and temperature buildup resulting from a fast charging current in overcharge will destroy most batteries in a short time, unless the fast charging current is terminated. Fast charging requires one hour, or less. Specially designed sealed nickel-cadmium batteries that can be quick, or in some cases fast, charged without charge control are too expensive for many applications.
A number of battery parameters, namely, terminal voltage, internal pressure, and temperature, change as a function of the state of charge of a sealed nickel-cadmium battery. When charge control is provided for fast charging a sealed nickel-cadmium battery, one or more of these parameters is sensed to determine when the battery is fully charged and the charging current should be terminated. One method of charge control, involving so-called voltage cutoff (VCO), determines when the battery is in overcharge by sensing the rapid rise in battery terminal voltage as full battery charge is reached. Very accurate sensing and tight tolerances are required where control is based upon only one parameter. For example, it is difficult to sense the terminal voltage alone because the voltage-charge characteristic is temperature dependent, particularly the rate of rise and the peak voltage value. The most accurate technique to date for sensing when the battery is in overcharge, called voltage-temperature cutoff (VTCO), is based upon both battery temperature and terminal voltage. Great care must also be taken to properly match the resistances of the cells connected in series to form a battery. If the cells are mismatched, one or more cells may become damaged before the charge control terminates charging. The problems attendant upon accurately sensing when a nickel-cadmium battery is fully charged have raised the cost of fast charging systems to the point of precluding their use in many commercial and industrial products.